1. Technical Field of the Invention
The present invention relates to an electrophoretic display, a method and apparatus for driving it, and an electronic device using it.
2. Description of Related Art
In the conventional art, electrophoretic displays are known which consist of a pair of panels or substrates spaced apart in opposing relation, each of which is provided with an electrode. Between these electrodes a dyed dielectric fluid is provided. Suspended in the fluid are electrically charged particles having a pigment color different to the fluid in which they are suspended (hereinafter referred to simply as pigment particles). In a display update operation, differing voltages are applied via a switching element to the electrodes to generate an electrostatic field in the dielectric fluid, causing the pigment particles to migrate in the direction of the applied field.
Electrophoretic displays utilizing an electrophoresis phenomenon are classed as non-luminous devices. In electrophoresis, pigment particles migrate under the action of Coulomb force which is generated when an electrostatic field is applied to a dielectric fluid in which the particles are dispersed.
However, prior art electrophoretic displays suffer from a problem in that they afford poor viewing characteristics. The present invention has been made to overcome this problem, and provides for the first time an active matrix electrophoretic display, which display has superior viewing characteristics.
As stated above, the object of the present invention is to provide an active matrix electrophoretic display. Also provided is a drive circuit integral to the device, and a method for driving the display by using the circuit. In addition there is provided is an electronic device attached to the electrophoretic display.
A method provided by the present invention is applied to an electrophoretic display comprising a common electrode, a plurality of pixels and a plurality of switching elements, each of which is assigned to a corresponding one of a plurality of switching elements. Each of the pixels is comprised of a pixel electrode which is connected to a corresponding switching element, with the pixel electrode being provided in spaced opposing relation to the common electrode, and a dispersal system comprising a colored fluid in which pigment particles are suspended being provided between the common electrode and the pixel electrode.
In the method of the present invention, a 1st voltage is applied to the common electrode. A 2nd voltage is then applied for a set period of time via a corresponding switching element to a pixel electrode, to generate an electrostatic field in the dispersal system of the pixel, to cause the pigment particles to migrate in the direction of the thus generated field to a desired position, which corresponds to a desired color gradation of the pixel. Next, the 1st voltage is applied via a corresponding switching element to the pixel electrode, to cancel the electrostatic filed and fix said pigment particles in a desired position.
In the present invention, in addition to these steps, which are common to the prior art, a new method is employed whereby differential voltages are applied which are calculated on the basis of a difference between a current average position of pigment particles and a subsequent desired position. By continually updating the voltage gradient using these parameters, positions of pigment particles can be updated without the need for an initialization step. Since no initialization step is required, display updates can be affected rapidly.
In the present invention, to further improve display image characteristics, it is preferable for there to be variations in the properties of pigment particles employed, such as charge and mass. As noted above, in the present invention pigment particles do not need to be initialized before a display update is made. This helps to overcome a problem which conventional electrophoretic displays suffer from, whereby after a voltage differential between electrodes is cancelled, pigment particles continue to move under their inertia. This residual movement of pigment particles causes fluctuations in an image displayed. In the case that minimal fluid resistance acts against pigment particles, inertial movement of the particles and resulting display fluctuations become pronounced. To overcome this problem of inertial particle movement, in the method of the present invention, after a differential voltage is applied to a second electrode, a further xe2x80x98brakexe2x80x99 voltage is applied to the dielectric fluid to stop movement of the pigment particles rapidly. Since a direction of motion of a particle is determined by a direction and polarity of an applied electrostatic field, a brake voltage to be applied has a polarity which is opposite to that of a voltage applied to a pixel electrode. Different from prior art displays, in the electrophoretic display of the present invention a plurality of discrete dispersal systems are employed in electrical communication with a common electrode. The dispersal systems comprise a colored dielectric fluid in which contrasting pigment particles are suspended; a plurality of data lines; a plurality of scanning lines; and a plurality of switching elements, which are provided at intersections between scanning and data lines. In addition, a plurality of pixel electrodes is also provided, and each of these pixel electrodes is connected to a corresponding switching element, and is also subject to a charge applied by the common electrode. In the method of driving the display of the present invention, a voltage is applied to the common electrode, and scanning lines are then subjected to sequential selection. In a next step, a voltage corresponding to a required screen update is applied by the pixel electrodes to the data lines, and a differential voltage is applied to the pixel electrodes via their respective switching elements, causing pigment particles suspended in the dielectric fluid of respective display systems to migrate in the direction of the applied field. To fix a position of the particles, a uniform voltage is then applied to respective pixel electrodes via their switching elements, and the switching elements are then turned off.
It is to be noted that in the present invention, voltages are applied as required, via switching elements, to respective pixel electrodes, thereby creating a matrix in the electrophoretic display In the method for driving the electrophoretic display of the present invention, each of the pixel electrodes is first subject to a preset uniform voltage applied by the common electrode. Scanning lines are then selected sequentially. Next, a voltage differential corresponding to a desired display update is applied via the switching elements to their respective pixel electrodes, whereby designated pigment particles are caused to migrate. To maintain a desired display state, a uniform voltage is applied to each of the pixel electrodes via respective switching elements, and, further, a break voltage is applied to counter inertial movement of the suspended pigment particles in each of the particle dispersion systems, and finally the switching elements are turned off.
According to the present invention, an active matrix electrophoretic display can be realized by applying differential voltages via a plurality of switching elements to a plurality of corresponding pixel electrodes.